1. Field of the Invention
The present invention relates to a fluid-measuring device and a fluid-measuring method, in particular, for measuring a flow of a gas, water, or the like with a flow sensor.
2. Description of the Related Art
A fluid-measuring device using a thermal flow sensor is well known for measuring a flow of a gas, water, or the like. Such a flow sensor utilizes a principle that a heater having a higher temperature than the fluid is disposed in the flow of the fluid, and a temperature profile of the fluid heated by the heater changes as a velocity of the flow of the fluid increases.
Japanese published patent application No. 2001-12988 discloses such a flow sensor. This conventional thermal flow sensor will be explained with reference to FIGS. 6 and 7.
In FIG. 6, a flow sensor 1 includes a silicon substrate 2, a diaphragm 3, micro heater 4 composed of platinum or the like formed on the diaphragm 3, a downstream side thermopile 5 disposed downstream of the micro heater 4 and formed on the diaphragm 3, power source terminals 6A, 6B for supplying driving current from a not shown power source to the micro heater 4, an upstream side thermopile 8 disposed upstream of the micro heater 4 and formed on the diaphragm 3, first output terminals 9A, 9B for outputting upstream side thermal signal from the upstream side thermopile 8, and second output terminals 7A, 7B for outputting a downstream side thermal signal from the downstream side thermopile 5.
Further, the flow sensor 1 includes: a right side thermopile 11 disposed substantially perpendicular to a flow of the fluid with respect to the micro heater 4 (a direction from P to Q), detecting physical properties data of the fluid, and outputting a right side thermal detecting signal (corresponding to a third thermal detecting signal); third output terminals 12A, 12B outputting the right side thermal detecting signal from the right side thermopile 11; a left side thermopile 13 disposed substantially perpendicular to the flow of the fluid with respect to the micro heater 4, detecting physical properties data of the fluid, and outputting a left side thermal detecting signal (corresponding to a third thermal detecting signal); fourth output terminals 14A, 14B outputting the left side thermal detecting signal from the left side thermopile 13; resistor 15, 16 for attaining a fluid temperature; and output terminals 17A, 17B for outputting a fluid temperature signal from the resistor 15, 16. The right and left side thermopiles 11, 13 compose a thermal sensor.
Each of the upstream side thermopile 8, the downstream side thermopile 5, the right side thermopile 11, and the left side thermopile 13 is composed of a thermocouple. This thermocouple is composed of p++-silicon and aluminum, and has a cold junction 5b, 8b and a hot junction 5a, 8a. When detecting heat, a thermal electromotive force is generated owing to a temperature difference between the cold junction 5b, 8b and the hot junction 5a, 8a so that the thermocouple outputs a temperature detecting signal.
Further, as shown in FIG. 7, the diaphragm 3 is formed on the silicon substrate 2. The hot junctions of the micro heater 4, the upstream side thermopile 8, the downstream side thermopile 5, the right side thermopile 11, and the left side thermopile 13 are formed on the diaphragm 3.
In the flow sensor 1, when the micro heater starts heating with the driving current from the outside, the heat generated by the micro heater 4 is transferred to the hot junctions 5a, 8a of the downstream side thermopile 5 and the upstream side thermopile 8 through the fluid. Because the cold junctions 5b, 8b are on the silicon substrate, they are at the substrate temperature. The hot junctions 5a, 8a are on the diaphragm 3 and heated by the transferred heat, and the temperature of the hot junctions 5a, 8a are hotter than the silicon substrate. Therefore, each thermopile generates the thermal electromotive force owing to the temperature difference between the hot junction 5a, 8a and the cold junction 5b, 8b, and outputs the temperature detecting signal.
The heat transferred by the fluid is transferred to the thermopiles by a synergistic effect of a thermal diffusion and a drift of the fluid from P to Q. Namely, when there is no drift, owing to the thermal diffusion, the heat is transferred to the upstream side thermopile 8 and the downstream side thermopile 5 equally, and a difference between an upstream side thermal signal from the upstream side thermopile 8 and a downstream side thermal signal from the downstream side thermopile 5 is zero.
On the other hand, when the drift is occurred, quantity of the heat transferred to the hot junction 5a of the downstream side thermopile 5 is increased, and quantity of the heat transferred to the hot junction 8a of the upstream side thermopile 8 is decreased. Therefore, the difference signal between the downstream side thermal signal and the upstream side thermal signal is a positive value corresponding to the velocity of the fluid.
When the micro heater 4 starts heating with the driving current, the heat generated by the micro heater 4 is transferred to the right side thermopile 11 disposed substantially perpendicular to the direction of the drift of the fluid with respect to the micro heater 4 by the thermal diffusion effect of the fluid without an influence of the drift of the fluid. The same heat is transferred to the left side thermo pile 13 disposed substantially perpendicular to the direction of the drift of the fluid with respect to the micro heater 4. Accordingly, the right side temperature detecting signal outputted from the third output terminals 12A, 12B owing to the electromotive force of the right side thermopile 11, and/or the left side temperature detecting signal outputted from the fourth output terminals 14A, 14B owing to the electromotive force of the left side thermopile 13 are corresponding to physical properties of the fluid such as thermal diffusion coefficient determined by conduction of heat, thermal diffusion, specific heat and the like. The proper process can attain physical properties.
A size of thermal diffusion constant also influences the upstream side temperature signal outputted by the upstream side thermopile 8 and the downstream side temperature signal outputted by the downstream side thermopile 5. They are changed similar to the left and right side thermopiles. Therefore, in principle, a correct flow rate of fluid having various thermal diffusion constants, namely, every fluid can be calculated by subtracting the right and/or left thermopile output from the upstream side temperature signal, the downstream side temperature, or the difference between them.
Therefore, a not-shown flow meter realizes a high accuracy measurement by calculating the physical properties of the fluid on the basis of the left and right side temperature detecting signals, and correcting the difference between the upstream and downstream side temperature signals with the physical properties.
However, such a flow sensor 1 has a problem that in spite of a correction corresponding to the physical properties of the fluid, reproducibility of a measurement is not good. In particular, in a case of measuring of large amount, namely, rapid drift, the reproducibility is worse. This is one reason of limiting a measuring range of the flow meter.
We found that the output of the flow meter changes even when the current is not supplied to the micro heater 4, namely, the flow sensor 1 is not driven. The detail will be explained as follows.
The flow rate is measured per 100 L/min in a standard condition.
A unit of an instrumental error as shown by a vertical axis in FIG. 9 is % RD (% of Reading). For example, in a meter having a maximum flow of 100 L/min, when the meter measures 10 L/min and the output is 9 L/min, the instrumental error is −10% RD. At this time, a tolerance is −1% (FS).
As shown in FIG. 8, when the thermal difference is zero and the heater is not heated, a temperature sensor output is V0. When the heater is heated, the temperature sensor output is V2. In this case, when a gas temperature becomes higher than a housing temperature, the output V0 turns to V1 and the output V2 turns to V3. However, because the flow sensor 1 is constantly powered, the output of V0 and V3 cannot be measured. The output V2 changes to V3.
FIG. 9 shows a measuring result of the output of the flow sensor 1 shown in FIG. 6. The instrumental errors are about +20% RD at the temperature difference of −30 degree, and about −20% RD at the temperature difference of +30 degree.
In the flow sensor 1 of FIG. 6, because the outputs of the upstream side thermopile 8 and the downstream side thermopile 5 caused by the temperature difference of the gas and the sensor body are substantially the same, the measurement of the differential output is automatically canceled. However, outputs of the left and right side thermopiles cannot be canceled because the differential output is not taken out. Because an accurate measurement cannot be taken if the differential output of the upstream side thermopile 8 and the downstream side thermopile 5 is corrected by the outputs of the right side thermopile 11 and the left side thermopile 13, the output accuracy of the flow sensor 1 becomes worse. Thus, the output of the flow sensor 1 is varied in proportion to the temperature difference between the gas and the sensor body.
Accordingly, an object of the present invention is to provide a fluid-measuring device and a fluid-measuring method for increasing a measuring accuracy without increasing complexity of a flow sensor structure.